Grace Monroe (Tembi Locke) is a regular character from Eureka, as well as a hidden villainess from "The Honeymooners," episode 5.10 of the series (airdate June 25, 2012).
Grace was introduced as a mechanic in the Season Four opener, "Founder's Day," but after the group returned to present day Eureka after an accidental trip to 1947, a new timeline was created where Grace was actually the wife of Henry Deacon, much to Henry's surprise. Henry later revealed the truth about their trip to 1947 and the different timelines, but after this, Grace and Henry became closer in their marriage as the series progressed. In the latter half of Season Four, Grace became the captain of the Astraeus mission, but in the beginning of Season Five, the mission was revealed as The Consortium's plan to control the entire group for their own use, only for Grace and the crew to be saved in "Force Quit."
In the aforementioned episode, "The Honeymooners," Henry and Jo Lupo discovered that Sheriff Cobb (Jack Carter's predecessor) was investigating a case regarding spies for The Consortium, with a hard drive revealing people connected to the group. Henry informed Grace about the drive, but unbeknownst to Henry, Grace was actually one of the conspirators, as she assisted Beverly Barlowe in stealing classified information. After learning about the investigation, Grace turned heel and destroyed the drive to cover up the truth, with the drive burning up while Henry attempted to play it. Later in the episode, the evil Grace returned for the destroyed drive after hearing Vincent talk about the case and Henry's attempt to repair the drive, only to be caught by Henry, who--along with Jo--was shocked by Grace's heel persona.
Grace was taken to holding and arrested, and she revealed a shocking truth to Henry during their conversation: it was Henry (the alternate version) who introduced her to Beverly, leading to them working with her and the Consortium. She stated that she changed for the better when the current Henry returned to her, and worked to keep her past as a villainess behind her, while adding that she would face the consequences alone and keep quiet regarding Henry, despite Henry wanting to take responsibility for what his alternate timeline version had done. In the series finale, "Just Another Day," a fully redeemed Grace was released after Beverly turned over information about the group and the whereabouts of one of the masterminds, Michaela Wen.